The invention relates to apparatus for timing an event relative to clock signals, and in particular an event represented by the arrival of the front or leading edge of a pulse at a location provided with a clock.
To synchronize two or more clocks at different points on the surface of the Earth, e.g. atomic clocks, without moving the clocks, there has been a proposal to transmit one or more laser pulses from each site to a common target placed so that it can "see" at least two of the sites at the same time. Laser pulses can be transmitted at instants which are very precisely known relative to the clock on the transmitter site, and the time interval between the arrivals of laser pulses from two different clocks can be measured. The target is advantageously an Earth satellite.
The time interval is measured by timing the arrivals of the respective laser pulses at the satellite using an on board clock, and in determining the difference between said arrival times. The difference may be determined on the ground on the basis of data transmitted from the satellite concerning the respective arrival times.
In this application, the timing must be exceptionally accurate: to within better than one nanosecond.
Preferred embodiments of the present invention may be carried on board a satellite and used to time events such as the arrival of a laser pulse at the satellite to within better than one nanosecond.